I've found the FRBs are the cheapest way to mail brass. Recently mailed off 2500 9mm cases in a medium, and have 2k 223 ready to go in a large. I'm wary of sending stuff that heavy with out reinforcing it, so I take mtgianni's method a step further, and build a box in the box. Lots of fiber tape, and make sure the load doesn't slide around. So far, so good. I think you can fit a couple hundred cases in a small, which wouldn't need all the reinforcing, just the tape.